Mobile WiMAX IEEE 802.16e standard has added mobility support to its predecessor IEEE 802.16d. Handover (HO) of a Mobile Subscriber Station (MSS) between WiMAX Base Stations (BSs) takes place in a defined sequence of events as shown in the FIG. 1.
A cell reselection process 102 is a periodic process in which MOB_NRB-ADV of mobile neighborhood advertisement messages from a serving BS are collected in a MMS. The MOB_NRB-ADV message contains parameters of neighboring BSs to synchronize in case of possible HO. Once the MSS constructs a virtual network topology using MOB_NRB-ADV message, the MSS starts scanning the neighbor BSs in pre-assigned period designated by the serving BS using MOB_SCN-REQ and MOB_SCN-RSP messages The MSS provides a set of target BSs in the MOB_SCN-REQ message to the serving BS, and the serving BS then negotiates with all or selected target BSs based on triggering events set in UCD (uplink channel descriptor) and DCD (downlink channel descriptor) messages. Through backbone network the serving BS collects rendezvous time allocated by each target BS. This report is sent to the MSS using MOB_SCN-RSP message. At the rendezvous time or scanning time, the MSS scans either one or more target BSs However, the MSS may scan all the BSs in the list of the MOB_SCN-RSP or few target BSs based on triggering event.
During scanning, after a target BS's ID is confirmed, the MSS tries to synchronize with its downlink transmission 106 and estimate the quality of the PHY channel. The synchronization happens if association or an optional initial ranging is done during scanning, else it is done only after HO decision 104.
The function of the association is to enable the MSS to acquire and record ranging parameters and service availability information for the purpose of proper selection of a target BS to serve the HO and/or expediting a potential future HO to the target BS. Recorded ranging parameters of an associated BS may be further used for setting initial ranging values in future ranging events during actual HO. The association parameters (e.g., PHY offsets, CIDs, etc.) stored in the MSS are obtained from RNG-RSP of ranging response message of the target BS.
In the process of the HO decision and initiation 104, the HO begins with the decision that may originate either at the MSS, the serving BS or on the network. The HO proceeds with the notification of either MOB_MSHO-REQ or MOB_BSHO-REQ A response messages MOB_BSHO-RSP holds a list of the targets BS that is compiled by the serving BS based on the possible MSS performance computed in the network. The serving BS negotiates with the possible targets BSs in the list given in the MOB_MSHO-REQ message The message MOB_BSHO-RSP contains the rendezvous time and opportunity for contention free coordinated ranging processing with the possible target BSs. During the rendezvous time either obtained from the association table in the MSS or in the MOB_MSHO-RSP message, the MSS synchronizes with the downlink transmission of possible target BSs 106.
Once the MSS synchronizes with the downlink transmission of a target BS, the MSS scans the target BS for UL-MAP that includes either a contention- or non-contention-based MSS Initial Ranging opportunity In either case, the MSS sends RNG-REQ message that may include MSS MAC Address or HO_ID assigned in MOB_BSHO-REQ or MOB_BSHO-RSP messages The RNG-REQ message contains serving BSID and Ranging Purpose Indication. On successful Ranging, the target BS assigns to the MSS Basic CID and Primary CID in the RNG-RSP management message 108. The reception of the RNG-RSP reception indicates successful network re-entry whereby the MSS sends a Bandwidth Request header to the serving BS.
Capability negotiation and registration process establishes security, authorization, authentication and service flow remapping. For security functionality, the MSS issues HMAC/CMAC Tuple as the last message item in the RNG-REQ management message using the Authorization Key and Key Sequence Number derived for use on the target BS. The remapping of service flow can be given to the MSS by the target BS in a REG-RSP provided that the target BS has obtained a backbone message containing network to build 110.
After the handover request/response handshake has been completed, the MSS begins the actual HO. At some stage during the HO process, the MSS terminates service with the serving BS. This is accomplished by sending a MOB_HO-IND message with the HO-IND_type=0b00 value indicating serving BS release 112. The serving BS starts the Resource retain timer from value Resource_Retain_Time. The serving BS retains the connections, MAC state machine and PDUs associated with the MSS for service continuation until the expiration of the timer.
HO cancellation 114 may happen in two different ways. First, when an MSS transmits MOB_HO-IND message with HO cancel option (HO_IND_type=0b01), and second, a drop where an MSS stops communicating with its serving BS before the normal HO sequence (outlined in Cell Selection and Termination) with the serving BS has been completed. In either case, the MSS can send HO_IND_type=0b01, and if the serving BS receives the message before the Resource Retain Timer has expired, the MSS and serving BS immediately starts normal operation.
However, in the existing Mobile WiMAX system with the HO standard briefly discussed above, there are a number of logical changes introduced in the HO algorithm. The current standard facilitates seamless HO but does not execute HO in timely fashion as required by IEEE 802.16m. More specifically, during the HO process, the serving BS directs the target BSs to reserve resources for likely HO of an MSS to one of the target BSs, but as the MSS decides on a particular target BS, rest of the target BSs are requested by the serving BS to release the resources Reservation of the resources is a costly affair when considering some of the bottleneck resources in a BS. Such resources may be bandwidth, code, etc.
Therefore, there exists a need to achieve the IEEE 802.16m standard requirements for the HO.